Interconnect structure having large self-aligned vias

ABSTRACT

A wavy line interconnect structure that accommodates small metal lines and enlarged diameter vias is disclosed. The enlarged diameter vias can be formed using a self-aligned dual damascene process without the need for a separate via lithography mask. The enlarged diameter vias make direct contact with at least three sides of the underlying metal lines, and can be aligned asymmetrically with respect to the metal line to increase the packing density of the metal pattern. The resulting vias have an aspect ratio that is relatively easy to fill, while the larger via footprint provides low via resistance. An interconnect structure having enlarged diameter vias can also feature air gaps to reduce the chance of dielectric breakdown. By allowing the via footprint to exceed the minimum size of the metal line width, a path is cleared for further process generations to continue shrinking metal lines to dimensions below 10 nm.

CROSS REFERENCE TO RELATED APPLICATION

The present patent application is a continuation-in-part of U.S. patentapplication Ser. No. 14/231,448 filed on Mar. 31, 2014.

BACKGROUND

1. Technical Field

The present disclosure generally relates to high speed integratedcircuits, and in particular, to the fabrication of interconnectstructures using a self-aligned dual damascene process.

2. Description of the Related Art

As integrated circuit technology advances, building metal interconnectstructures that are used to wire transistors together becomes more andmore challenging. Metal lines can assume a variety of different shapes,from straight wires to cells made up of intertwined C-shapes. Regardlessof their shapes, design rules for metal lines are typically based onscaling a pitch dimension that assumes a regular pattern of equal linewidths and spacings between the metal lines at each metal layer. Metalline widths are generally expected to shrink with every new processgeneration to further improve integrated circuit performance.

Depending on the type of process integration scheme used, viasconnecting stacked metal lines vertically to one another are constrainedto be smaller than the metal lines in order for the via footprint to besurrounded by metal. Such a constraint exists, for example, when formingvias according to a self-aligned dual damascene process that avoids aseparate via lithography step. Thus, as the metal line design widthsshrink with each process generation, the self-aligned vias shrink aswell. However, smaller vias incur higher via resistance, causing RCdelays to worsen. In addition, smaller via footprints cause the viaaspect ratio, i.e., the ratio of height to width, to increase, makingthe vias taller and narrower, and therefore more difficult to fill withmetal. Incomplete via fill in turn degrades reliability by causing opencircuit failures, for example. Thus, for multiple reasons, it isadvantageous for via footprints to remain large, while metal linescontinue to shrink with each technology generation.

BRIEF SUMMARY

An interconnect structure having wavy metal lines allows via footprintsto exceed the nominal width of metal line portions connecting them. Inone embodiment, the resulting metal line profile shape resembles abicycle chain. In another embodiment, the metal line resembles a linearseries of T-shaped unit cells, in which the width of the metal linealternates between a narrow value, an intermediate value, and a widevalue. The wide value corresponds to a large via landing pad for viasforming an electrical connection to the next highest metal layer. Toachieve greater lateral packing density, adjacent wavy metal lines canbe staggered so that the via landing pads on one metal line are next tothe narrow metal connecting portions of neighboring metal lines. Bylifting the shrink constraint for vias, thereby allowing the viafootprint to exceed the minimum size of the metal line width, a path iscleared for further process generations to continue shrinking metallines to dimensions below 10 nm.

A wavy line interconnect structure can be formed using a dual damasceneprocess in which vias are self-aligned without the need for a dedicatedlithography mask. Instead, metal line trenches are lined with asacrificial layer that completely fills the narrow spaces whileunder-filling the wide spaces that correspond to via locations. Thesacrificial layer thus acts as a via hard mask, allowing etching of theunderlying thick dielectric block, while protecting narrow features ofthe trenches that correspond to the metal line interconnects. Theresulting vias have an aspect ratio that is relatively easy to fill,while the larger footprint provides low via resistance.

One way to pattern wavy metal lines using conventional opticallithography uses a mask cell design that includes a series ofrectangular opaque features diagonally offset from one another along anaxis. Via landing pads are then added at one end of the mask cell byplacing two rectangular features adjacent to one another. The methoddisclosed takes advantage of the shortcomings of optical lithography toachieve the desired wavy line pattern by anticipating diffraction errorsthat will occur during exposure of the rectangular features. Such atechnique is generally known in the art as optical proximity correction(OPC). Typically, OPC is used to produce square corners where thestandard lithography tends to round off the corners of a circuitpattern. In this case, however, the rounding effect is intentional.During the patterning process, corners of the rectangular features willtransfer to the photoresist and the metal trench as rounded features.Meanwhile, the offset rectangular features, instead of remainingdistinct, will join together to approximate the desired undulating wavyline shape. In similar fashion, each rectangle pair will transfer to themetal trench layer as a combined shape, forming a single large landingpad to accommodate the vias.

According to one embodiment, large vias are designed to land on narrowportions of the wavy metal line instead of, or in addition to, vias thatland on the designated landing pads. Normally, when a large viafootprint exceeds the size of an underlying metal landing pad, theelectrical contact area at the interface is reduced, thereby causing anincrease in via resistance. However, if the large via is formed so thatthe via depth extends to the bottom of the metal line instead ofstopping on the top surface of the metal line, an enlarged diameter viais formed that operates with significantly less via resistance.Furthermore, the enlarged diameter via need not be centered on the metalline, but can be aligned asymmetrically with respect to the metal line.

According to one embodiment, an interconnect structure featuringenlarged diameter vias includes a two-component inter-layer dielectric.The two-component ILD includes a bottom dielectric material adjacent tothe metal line, and a top dielectric material adjacent to the via. Useof two different materials allows greater flexibility in the circuitdesign of the overall interconnect structure.

According to one embodiment, adjacent enlarged diameter vias areeffectively encapsulated by substituting a silicon carbide-basedinsulator, e.g., SiC or SiCOH for the low-k ILD materials. Use of such aSiC-based insulator offers additional protection against dielectricbreakdown. Forming an air gap within the SiC-based ILD then reducescapacitance between the adjacent enlarged diameter vias to compensatefor the increased capacitance of the encapsulating material.

According to one embodiment, an enlarged diameter via surrounds theunderlying metal line on all four sides, further increasing theelectrical contact area to achieve an additional performance benefit.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elements.The sizes and relative positions of elements in the drawings are notnecessarily drawn to scale.

FIG. 1A is derived from a top-down scanning electron micrograph of aconventional array of metal lines, according to the prior art.

FIG. 1B is a layout diagram of adjacent narrow metal lines encompassinglarge vias according to a first embodiment in which the metal linesresemble a bicycle chain.

FIG. 2A is a layout diagram of an optical lithography mask designshowing regions of chrome on a glass mask plate that can be used to formdiagonal wavy metal lines according to a second embodiment.

FIG. 2B is derived from a top-down scanning electron micrograph of astructure as made when the mask has an array of rectangular-shaped viasthat create diagonal wavy metal lines having narrow, intermediate, andwide regions, the wide regions accommodating large vias.

FIG. 2C is a schematic of a template corresponding to the unit cell thatdescribes the via shape arrangement shown in FIG. 2B.

FIGS. 2D, 2E, and 2F show a layout diagram, a top-down scanning electronmicrograph, and a unit cell template for creating vertically-orientedwavy metal lines, corresponding to FIGS. 2A, 2B, and 2C, respectively.

FIGS. 2G, 2H, 2J, and 2K are exemplary wafer maps showing a waferrotation sequence that facilitates patterning the wavy metal lines in aselected orientation.

FIG. 3 is a high-level flow diagram of a method of fabricating largeself-aligned vias in a dual damascene interconnect process, according toone embodiment.

FIG. 4A is a process flow diagram showing a detailed sequence ofprocessing steps that can be used to form wavy trenches using a wavyline mask, according to one embodiment.

FIG. 4B is a top plan view of wavy trenches after carrying outprocessing steps shown in FIG. 4A.

FIGS. 4C and 4D are cross-sectional views, along the cut lines shown, ofwavy trenches after carrying out processing steps shown in FIG. 4A.

FIG. 5A is a process flow diagram showing a detailed sequence ofprocessing steps that can be used to form a sacrificial layer for use asa hard mask, according to one embodiment.

FIG. 5B is a top plan view of wavy trenches after carrying outprocessing steps shown in FIG. 5A.

FIGS. 5C and 5D are cross-sectional views, along the cut lines shown, ofthe hard mask after carrying out processing steps shown in FIG. 5A.

FIG. 6A is a process flow diagram showing a detailed sequence ofprocessing steps that can be used to form large self-aligned vias,according to one embodiment.

FIG. 6B is a top plan view of wavy trenches after carrying outprocessing steps shown in FIG. 6A.

FIGS. 6C and 6D are cross-sectional views, along the cut lines shown, ofthe large self-aligned vias after carrying out processing steps shown inFIG. 6A.

FIG. 7A is a process flow diagram showing a detailed sequence ofprocessing steps that can be used to fill the wavy line trenches and thelarge self-aligned vias with metal, according to one embodiment.

FIG. 7B is a top plan view of wavy trenches after carrying outprocessing steps shown in FIG. 7A.

FIGS. 7C and 7D are cross-sectional views, along the cut lines shown, ofwavy metal lines and filled vias after carrying out processing stepsshown in FIG. 7A.

FIGS. 8A and 8B are derived from cross-sectional scanning electronmicrographs of a conventional interconnect structure containing highresistance vias and open vias respectively, according to the prior art.

FIGS. 9A, 9B, and 9C are cross-sectional views of an interconnectstructure during formation of enlarged diameter vias, according to oneembodiment.

FIGS. 10A and 10B are top plan views of metal lines onto whichfootprints of the enlarged diameter vias are superimposed, according toone embodiment described herein.

FIGS. 11A and 11B are cross-sectional views of an interconnect structureduring formation of enlarged diameter vias, according to one embodiment.

FIGS. 12A and 12B are cross-sectional views of an interconnect structurethat features enlarged diameter vias separated by an encapsulating ILDthat includes air gaps.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various aspects of thedisclosed subject matter. However, the disclosed subject matter may bepracticed without these specific details. In some instances, well-knownstructures and methods of semiconductor processing comprisingembodiments of the subject matter disclosed herein have not beendescribed in detail to avoid obscuring the descriptions of other aspectsof the present disclosure.

Unless the context requires otherwise, throughout the specification andclaims that follow, the word “comprise” and variations thereof, such as“comprises” and “comprising” are to be construed in an open, inclusivesense, that is, as “including, but not limited to.”

Reference throughout the specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. Thus, the appearance of the phrases “in oneembodiment” or “in an embodiment” in various places throughout thespecification are not necessarily all referring to the same aspect.Furthermore, the particular features, structures, or characteristics maybe combined in any suitable manner in one or more aspects of the presentdisclosure.

Reference throughout the specification to integrated circuits isgenerally intended to include integrated circuit components built onsemiconducting substrates, whether or not the components are coupledtogether into a circuit or able to be interconnected. Throughout thespecification, the term “layer” is used in its broadest sense to includea thin film, a cap, or the like and one layer may be composed ofmultiple sub-layers.

Reference throughout the specification to conventional thin filmdeposition techniques for depositing silicon nitride, silicon dioxide,metals, or similar materials include such processes as chemical vapordeposition (CVD), low-pressure chemical vapor deposition (LPCVD), metalorganic chemical vapor deposition (MOCVD), plasma-enhanced chemicalvapor deposition (PECVD), plasma vapor deposition (PVD), atomic layerdeposition (ALD), molecular beam epitaxy (MBE), electroplating,electro-less plating, and the like. Specific embodiments are describedherein with reference to examples of such processes. However, thepresent disclosure and the reference to certain deposition techniquesshould not be limited to those described. For example, in somecircumstances, a description that references CVD may alternatively bedone using PVD, or a description that specifies electroplating mayalternatively be accomplished using electro-less plating. Furthermore,reference to conventional techniques of thin film formation may includegrowing a film in situ. For example, in some embodiments, controlledgrowth of an oxide to a desired thickness can be achieved by exposing asilicon surface to oxygen gas or to moisture in a heated chamber.

Reference throughout the specification to conventional photolithographytechniques, known in the art of semiconductor fabrication for patterningvarious thin films, includes a spin-expose-develop process sequencetypically followed by an etch process. Alternatively or additionally,photoresist can be used to pattern a hard mask (e.g., a silicon nitridehard mask), which, in turn, can be used to pattern an underlying film.

Reference throughout the specification to conventional etchingtechniques known in the art of semiconductor fabrication for selectiveremoval of polysilicon, silicon nitride, silicon dioxide, metals,photoresist, polyimide, or similar materials includes such processes aswet chemical etching, reactive ion (plasma) etching (RIE), washing, wetcleaning, pre-cleaning, spray cleaning, chemical-mechanicalplanarization (CMP) and the like. Specific embodiments are describedherein with reference to examples of such processes. However, thepresent disclosure and the reference to certain deposition techniquesshould not be limited to those described. In some instances, two suchtechniques may be interchangeable. For example, stripping photoresistmay entail immersing a sample in a wet chemical bath or, alternatively,spraying wet chemicals directly onto the sample.

Specific embodiments are described herein with reference to interconnectstructures that have been produced; however, the present disclosure andthe reference to certain materials, dimensions, and the details andordering of processing steps are exemplary and should not be limited tothose shown.

Turning now to the Figures, FIG. 1A shows a top plan view of aconventional array 90 of metal lines 92 inlaid in a dielectric block 94.Such an array characterized by a pitch 96 of 40 nm or less typically isfabricated of copper or a copper alloy using a dual damascene process,as is well known in the art [see, for example, “Silicon Processing forthe VLSI Era,” Vol. 4, p. 674-679, by Stanley Wolf]. In short, a dualdamascene process entails first forming a trench in the dielectric block94, and then forming a via that extends from the bottom of the trench toan underlying metal layer. The trench and the via are then filled withmetal together in one step. Thus, each interconnect layer entailsperforming two lithography steps and one metal deposition step. One ofthe two lithography steps can be eliminated by employing a self-alignedvia process in which a sacrificial layer is used as a hard mask.Self-aligned dual damascene vias are also well known in the art and canbe understood by referencing the Wolf article cited above. Viasunderlying the metal lines 92 are not shown in FIG. 1A because they haveequal or smaller widths than the metal lines and they are thereforeobscured by the metal lines.

With reference to FIG. 1B, for the reasons explained above, it isdesirable to form wavy metal lines 102 shaped like bicycle chainstructures 100 in which wide portions 104 accommodate via landing pads106 spaced periodically between narrow portions 105. Such aconfiguration allows the via landing pads 106 to exceed the width of thenarrow portions 105. Adjacent metal lines 102 a and 102 b can bestaggered relative to one another so that the via landing pads 106located at the wide portions 104 along 102 a are aligned with the narrowportions 105 of neighboring wavy metal line 102 b. Thus, the wavy metallines can maintain a constant spacing 108. The bicycle chain structures100 are thus one exemplary embodiment of a wavy metal line design thataccommodates large vias.

A second exemplary embodiment of such a wavy metal line design is shownin FIGS. 2A, 2B, and 2C. FIG. 2A shows an optical lithography wavy linemask design 110 that results in an array of wavy metal lines structures120 shown in FIG. 2B. The wavy metal line structures 120 include aplurality of narrow portions 122, intermediate width portions 123, andwide portions 124, along each wavy metal line. In one embodiment, thenarrow portions 122 are in the range of about 5-30 nm wide, with anominal pitch of 48 nm. Dimensions of the wide portions 124 are withinthe range of about 10-40 nm. Wavy metal lines having about three or morenarrow portions 122 for every wide portion 124 maintain an average linewidth that is slightly larger than the width of the narrow portion 122.Again, adjacent metal lines can be staggered so the wide portions 124 ofone line are placed next to the intermediate or narrow portions 123, 122of neighboring lines.

The wavy metal line structures 120 are achieved by approximating theportions of varying widths using rectangular mask features in the wavyline mask design 110. For example, the wavy line mask design 110 can beimplemented by forming rectangular opaque features 112 and 114 on areticle made of chrome on a glass plate 113. When the reticle is exposedto light in an optical lithography stepper or scanner, the light isblocked by the rectangular opaque features 112 and 114. However,diffraction effects cause the pattern of rectangles to become blurred atthe target surface, thereby producing the wavy-line patterns shown inFIG. 2B.

The optical lithography wavy line mask design 110 (FIG. 2A) can bethought of as an optical lithography mask cell 130 (FIG. 2C) replicatedthroughout a region of a larger optical lithography reticle that mayinclude many different mask designs. The optical lithography mask cell130, shown in FIG. 2C, is then a fundamental building block of the wavyline mask design 110 shown in FIG. 2A. The optical lithography mask cell130 shows more clearly the relative orientations of the rectangularopaque features 112 and 114. The rectangular opaque features 112 areisolated rectangles, offset from one another by a first offset distance116. Pairs of rectangular opaque features 114 touch one another at leastat a contact point 118, and the pairs are then offset from therectangular opaque features 112 by a second offset distance 117.Furthermore, the contact point 118 and center points 131 of therectangular opaque features 112 are aligned along an axis 132. When atemplate 134 of the mask cell 130 is overlaid onto the wavy metal linestructures 120, the various elements of the metal line can more clearlybe associated with corresponding mask features that define them. It isnoted that the via landing pads 106 that correspond to the wide portions124 are formed by the pairs of rectangular opaque features 114, whilethe narrow portions 122 of the metal lines correspond to locations whereadjacent rectangular features blur together.

Intentional manipulation of mask features to compensate for shortcomingsin lithographic capability is well known in the art of photolithographyas Optical Proximity Correction (OPC). OPC is often used to patternsquare corners that would otherwise become rounded by diffractioneffects. In the present case, however, the rounded features forming thewavy metal line structures 120 are intentionally created by leveragingdistortion that occurs in the lithography process, as opposed tocorrecting for the distortion. While it is true that conventional metallines are not all as uniform and straight as those shown in FIG. 1A, theresulting wavy metal line structures 120 are unique compared with knownexisting metal line shapes as shown in documentation describing SRAMcells associated with various microprocessor chips, e.g., those of TexasInstruments OMAP™, TMX320x, D6298x, and the like.

If it is desired to produce a wavy line pattern that is orientedhorizontally or vertically instead of diagonally, simply rearranging therectangular opaque features 112 along a horizontal or vertical axisinstead of along the oblique axis 132 may not result in the desired wavyline pattern. One way to change the wavy line orientation is to rotatethe wavy line mask design 110 to produce, for example, a vertical wavyline mask design 110A as shown in FIG. 2D. Use of vertical wavy linemask design 110A results in an array of wavy metal line structures 120Ashown in FIG. 2E, for which a corresponding optical lithography maskcell 130A (FIG. 2F) is oriented along a vertical axis 132A. However, itmay not be feasible or cost-effective to create a new mask design andfabricate a new reticle for each desired orientation. Therefore, analternative method to produce wavy line patterns having any selectedorientation, using the same oblique wavy line mask design 110, isdescribed below.

FIGS. 2G, 2H, 2J, and 2K show a series of top plan views of asemiconductor wafer 140, illustrating an exemplary sequence forimplementing the oblique wavy line mask design 110 so as to produce wavymetal lines that are oriented at any selected angle. In the exampleshown, the wafer 140 includes 52 dice 144 that are arranged in a grid.To pattern the dice with wavy metal lines that are oriented horizontallyas shown in FIG. 2K, using the oblique wavy line mask design 110, eachwafer is rotated before and after photolithography processing. Forexample, FIG. 2H shows the wafer 140 after rotating clockwise by 45degrees as indicated by the position of a wafer notch 142. In FIG. 2J,the wafer 140 is then exposed to light through the oblique wavy linemask design 110 wherein the oblique axis 132 of the mask cell 130 isindicated on each die 144. Following lithography processing, the wafer140 is counter-rotated back to its original notch-down position as shownin FIG. 2K, in which the wavy lines are shown in the desired horizontalorientation with respect to the notch 142. In another example, if apattern of vertical wavy lines is desired, the wafer 140 undergoes a 45degree counterclockwise rotation prior to the lithography operation, anda 45 degree clockwise rotation after the lithography operation. Insimilar fashion, the rotation angle can be altered to achieve anydesired orientation of the wavy metal lines.

FIG. 3 shows general steps in a method 150 of fabricating wavy metallines having large vias. The method 150 is one embodiment of aself-aligned dual damascene process for fabricating an interconnectstructure, i.e., a network of wavy metal lines and large vias, thatprovides electrical connections for transistors within integratedcircuits. Metal lines within the interconnect structure have the wavyline pattern of the wavy metal line structures 120 shown in FIG. 2B.

At 152, wavy trenches are formed in a dielectric block.

At 156, large self-aligned vias are formed in the dielectric block,extending away from the wavy trenches.

At 158, the wavy trenches and the large self-aligned vias are filledwith metal in the same process step.

FIGS. 4A-7D show and describe in further detail steps in the method 150.In each set of Figures A-D, A is a detailed flow diagram; B is a topplan view showing at least a wide portion 104 and a narrow portion 105of a wavy metal line 102 after performing the steps shown in A; C is across-sectional schematic view through the wide portion 104 thataccommodates a via; and D is a cross-sectional view through the narrowportion 105. In accordance with convention, arrows on each cut linerepresent the direction of an observer's eye looking at thecorresponding cut plane.

FIGS. 4A-4D show and describe patterning the wavy line trenches in adielectric block, at step 152, according to one embodiment. Formation ofthe wavy line trenches is the only process step in the method 150 thatentails use of photolithography. The trench formation begins with adielectric block 104 that has been deposited onto an underlying metallayer, e.g., metal 1. The dielectric block 157 is made of silicondioxide or an ultra-low-k (ULK) dielectric material, as is well known inthe art. The underlying metal layer includes a metal liner 151 made of,for example, Ti, TiN, or TaN; a layer of bulk metal 153, e.g., copper;and a metal cap layer 155, e.g., silicon nitride carbide (SiNC), as arewell known in the art. It is noted that the trench extends in adirection perpendicular to the underlying metal layer.

At 160, photoresist is applied to the thick inter-layer dielectric (ILD)block 157. Applying the photoresist entails first applying an opticalplanarization layer (OPL) 159, followed by an anti-reflection coating(ARC) 161, and finally the actual photoresist 163. The OPL 159 is usedto fine-tune planarization of the surface of the dielectric block 157prior to exposure, in order to reduce distortion of the pattern. The OPL159 is made of, for example, a spin-on glass (SOG), as is known in theart.

At 162, the photoresist 163 is patterned by exposure to light through amask fabricated according to the wavy line mask design 110, followed bytreatment with a developer that removes portions of the photoresist 163.

At 164, a wavy line trench 166 is etched in the dielectric block 157down to a trench depth 167. The wavy line trench 166 includes a wideportion of width D where a large via 169 will be formed, as shown inFIG. 4C, and a narrow portion of width d, that corresponds to thenominal metal line width, e.g., 24 nm, as shown in FIG. 4D.

FIGS. 5A-5D show and describe the formation of a sacrificial, ordisposable, layer, according to one embodiment.

At 170, a sacrificial layer 171 is deposited into the wavy line trench166. In one embodiment, the sacrificial layer 171 is made of a materialsuch as polysilicon, low-temperature silicon dioxide, or SiN. Thesacrificial layer 171 overfills the narrow portion of the trench (d) toa narrow portion thickness 172 having a notch 178 therein, whileconformally underfilling the wide portion of the trench (D) to a wideportion thickness 175. Thus, as shown in FIGS. 5C and 5D, the narrowportion d of the trench is entirely blocked, while the wide portion Dcontains a hard mask that will define a large via having a via width176.

The via width 176 is much greater than the narrow portion, d, of themetal line, which eases formation of the overall interconnect structure.The significance of the independence of the via width from the nominalmetal line width d is that a path is cleared for the nominal metal linewidth in future technology generations to shrink without causing viafailures. The metal line widths can continue getting smaller and smallerwith each technology generation while the via size remains the same. Thenarrow portion thickness 172 is larger than d/2, but smaller than D/2.By using the sacrificial layer 171 as a hard mask, a via lithographystep is avoided. Via formation without the need for a separate masklayer is referred to by those skilled in the art as a self-aligned via.

In another embodiment, the sacrificial layer 171 is made of a directself-aligned (DSA) polymer material. A DSA polymer sacrificial layer 171fills the narrow portions (d) of the wavy line trench so that the narrowportion thickness 172 is negligible. Likewise, the DSA polymersacrificial layer 171 deposits on the sidewalls of the wide portions (D)to form a hard mask having a negligible wide portion thickness 175.Consequently, use of the DSA polymer as the sacrificial layer 171simplifies the etch process, as is shown below.

FIGS. 6A-6D show and describe the formation of vias at step 156,according to one embodiment.

At 180, the sacrificial layer 171 is etched to expose a via landing pad181 at the surface of the underlying bulk metal 153. A first anisotropicetch step removes the sacrificial layer 171 from the bottom of the wavyline trench 166, down to the trench depth 167. If a DSA polymer is usedas the sacrificial layer 171, the first anisotropic etch step is notneeded because the bottom of the trench is already exposed.

At 182, a second anisotropic etch continues removing dielectric materialin a downward direction to the underlying three-metal layer, creatingthe large via 169. Finally, a third etch step removes the cap layer 155to expose the bulk metal 153, as shown in FIG. 6C. Meanwhile, as shownin FIG. 6D, the sacrificial layer 171 protects the narrow portion of thewavy line trench 166.

FIGS. 7A-7D show and describe the trench and via fill at step 158,according to one embodiment.

At 190, the sacrificial layer 171 is removed using an etch chemistrythat has a high selectivity to the dielectric block 157. For example, ifthe sacrificial layer 171 is SiN and the dielectric 157 is silicondioxide, the sacrificial layer 171 can be removed using a phosphoricacid dip.

At 192, the wavy line trench 166 and the large via 169 are filled withmetal. The metal fill process includes first depositing a metal liner195, e.g., titanium (Ti), titanium nitride (TiN), tantalum nitride(TaN), titanium carbide (TiC), cobalt (Co), rubidium (Ru), andcombinations thereof; followed by deposition of a bulk metal 197, e.g.,aluminum (Al), copper (Cu), tungsten (W), cobalt (Co), nickel silicide(NiSi), cobalt silicide (CoSi), and combinations thereof, whereincombinations include metal laminates, alloys, and the like.

At 194, a chemical mechanical planarization (CMP) step is performed toplanarize the metal liner 195 and bulk metal 197 to the height of thedielectric block 157.

The resulting metal lines shown in FIG. 7B are the wavy metal linestructures 120 shown in FIG. 2B, including the narrow portions 122 ofwidth d, intermediate portions 123, and wide portions 124 of width D,accommodating the large via 169. If, for example, the metal lines thusformed are at metal 2, the wide portions 124 serve as large via landingpads 106 for subsequently formed large vias that will later connect toupper metal lines, e.g., metal 3. It is noted that the large via 169 hasan aspect ratio of about 3:1 or less, which can easily be filled withmetal compared with existing vias that present more challenging aspectratios, in the range of about 4:1-5:1.

FIGS. 8A and 8B show examples of malformed high aspect ratio vias. FIG.8A shows a first cross-sectional view 200 of a row of high aspect ratiovias that includes a void 202 due to incomplete via fill. The void 202will result in a high via resistance. FIG. 8B shows a secondcross-sectional view 203 of a row of high aspect ratio vias thatincludes several incompletely formed vias, for example, 204. Suchincomplete formation occurs when the etch process fails to punch throughto the underlying metal layer due to excess sidewall polymer formation.The incompletely formed vias 204 will result in open circuits. Suchexamples of malformed vias can be prevented by reducing the aspect ratiousing enlarged diameter vias that are easier to etch and fill.

FIGS. 9A, 9B, and 9C show cross-sectional views at successive stepsduring the formation of enlarged diameter vias according to oneembodiment of a dual damascene process. FIG. 9A shows three exemplarymetal lines 205 in a first metal layer having a pitch 206. The metallines 205 are inlaid in a base ULK dielectric material 207. Each metalline includes the metal liner 151, the bulk metal layer 153, and themetal cap layer 155. Each one of the metal lines 205 has a width in therange of about 10-60 nm, with a target width of 24 nm.

With reference to FIG. 9B, following formation of the metal lines 205, aULK inter-layer dielectric 208 is formed on top of the first metallayer. The metal cap layer 155 separates the base ULK dielectric 207from the inter-layer dielectric 208. FIG. 9B shows a cross-sectionalview after etching symmetric enlarged diameter via openings 212 a thatare centered with respect to the metal lines 205. Instead of landing ona top surface 213 of the metal lines 205, as is the usual practice informing vias, the enlarged diameter via openings 212 a extend to thelower boundary 214 of the metal lines 205, thus exposing three surfacesof each one of the metal lines 205. Etching the enlarged diameter viaopenings can be timed to stop on at the lower boundary 214 by use of anadvanced process control (APC) scheme. Using APC, in-line film thicknessmeasurements are fed back to the etcher to adjust the etch time so thatan enlarged via depth 215 extends to the lower boundary 214. Theenlarged via depth 215, which includes a metal trench depth 217 and aconventional via depth 219 is in the range of about 20-150 nm, with atarget of 52 nm. About one third of the enlarged via depth 215 is equalto the first metal layer height, and about ⅔ of the enlarged via depth215 is above the first metal layer. The enlarged diameter via openings212 a have a pitch dimension within the range of about 10-64 nm with atarget pitch of about 48 nm. The via aspect ratio is thus only about1.5-2.0. ULK material separating the metal lines has an insulation width209, having a target value of about 16 nm.

FIG. 9C shows a cross-sectional view after filling the via openings 212a and forming a second metal layer 218. The second metal layer 218 isvertically spaced apart from the first metal layer by the inter-layerdielectric 208. The via fill includes a via liner 216 and a bulk viafill material which is desirably the same metal as that used to form thebulk metal layer 153. Following formation of the second metal layer 218,for example, by electroplating, a metal planarization process isperformed according to methods well known in the art. The completed dualdamascene structure is shown in FIG. 9C in which the enlarged diametervia wraps around three sides of the first metal line. As a result, avery large contact area is formed between the filled via and the firstmetal layer, including the top surfaces 213 of the metal line 205 aswell as both sides 217 of the metal line 205. Such a large contact areareduces the via resistance considerably, compared with a contact areafor a conventional via that includes only the top of the metal line 205.

FIGS. 10A and 10B show footprints of the enlarged via openings 212 a andlarge vias 169 according to two exemplary embodiments. The enlarged viaopenings 212 a are superimposed on the narrow portions 105 of the metallines 205, while landing pads 106 of large vias 169 are superimposed onthe wide portions 104. FIG. 10A corresponds to the embodiment shown inFIGS. 9A, 9B, and 9C described above in which the via openings 212 a arecentered on the metal lines 205 at the plane where the via openings 212a and the metal lines 205 intersect. The center-to-center distancebetween adjacent via openings 212 a corresponds to the pitch dimension206, and maintains the spacing 108 between adjacent metal lines, e.g.,102 a and 102 b. The enlarged diameter vias indicated by 212 a couplethe narrow portions 105 of the metal layer 205 to the next highest metallayer, e.g., a third metal layer, while the enlarged diameter viasindicated by the landing pads 106 couple the wide portions 104 to thenext lowest metal layer.

In the embodiment shown in FIG. 10B, enlarged via openings 212 b are notcentered on the metal lines 205. Instead, the via openings 212 b aresubstantially aligned asymmetrically, with one side of the narrowportions 105 of the metal lines 205, i.e., the via openings 212 b areoffset from the centers of the metal lines 205. In one embodiment, thevia openings 212 b are shifted laterally with respect to the metal lines205. In another embodiment, the via openings 212 b have a smallerdiameter and may or may not also be shifted laterally with respect tothe metal lines 205.

FIGS. 11A and 11B show cross sections of enlarged vias formed using abi-layer inter-layer dielectric, according to one embodiment. Thebi-layer film stack includes two different low k materials supportingthe first metal layer. Two such embodiments are shown in which the baseULK dielectric material 207 is replaced with a ULK bi-layer stack. TheULK bi-layer stack includes a first low-k material 222 and a secondlow-k material 224, such as, for example, a low-k silicon carbidehydroxide (SiCOH, a porous silicon carbide hydroxide (pSiCOH), aUV-cured porogen octomethylcyclotetrasiloxane (OMCTS), or a porogenlessUV-cured OMCTS. OMCTS and related materials are discussed in more detailin conference papers published in the Proceedings of the IITC 2005,paper 2-3, p. 9, by M. Fukusawa et al., and the Proceedings of the AMC,2005 paper 11.4 by K. Ida et al. The first and second low-k materialshave different dielectric constants k, however both of the low-kmaterials desirably have k values less than about 4.0. The first low-kmaterial 222 is located below the metal lines 205. The second low-kmaterial 224 has lower and upper surfaces at substantially the sameheights as the bottom and top surfaces of the metal lines 205. The firstlow-k material 222 is also deposited on top of the second low-k material224. The interface between the first low-k material 222 and the secondlow-k material 224 serves as a reactive ion etch (RIE) stop layer whenetching the enlarged via openings 212 a, so that the use of APC to timethe etch process becomes unnecessary. Use of the ULK stack also givescircuit designers additional options for crafting trench RC delays andvia RC delays. With two different ULK materials, designers can moreeasily modify the capacitance of the various layers via the k-values,wherein C=κ∈A/d. The enlarged vias shown in FIG. 11A are centered on themetal lines 205, whereas those shown in FIG. 11B are offset so that theyalign with the right hand walls of the metal lines 205.

FIGS. 12A and 12B show cross sections of interconnect structures thatfeature enlarged diameter vias that are electrically supported by theuse of an encapsulating ILD 226 containing air gaps 228. Two suchembodiments are shown. In both embodiments, the encapsulating ILD 226 isdesirably made of a dielectric material such as, for example, siliconcarbide (SiC) for better time dependent dielectric breakdown (TDDB)performance than is provided with a ULK material. Use of SiC thusprovides a higher reliability interconnect structure in which highvoltages are less likely to cause short circuits between adjacent metallines. Including air gaps 228 in the encapsulating ILD 226 provides alower capacitance ILD overall to compensate for the higher capacitanceof the SiC. The overall capacitance of the ILD including the air gaps228 is lower because the contribution of the air gaps to the dielectricconstant is at the lower limit of 1.0.

The tapered shape of the air gaps 228 provides greater structuralstability than is achieved using materials that incorporate air pocketsin a more random arrangement. Advantages of such tapered air gaps 228are described in greater detail in U.S. patent application Ser. No.14/098,286 entitled, “Advanced Interconnect with Air Gap” and Ser. No.14/098,346 entitled, “Trench Interconnect Having Reduced FringeCapacitance”. The air gaps 228 have a maximum width in the range ofabout 10-50 nm with a target width of about 20 nm. In one embodiment,the tapered air gaps 228 extend downward to the lower surface 214 of theunderlying metal lines 205.

In FIG. 12A, the enlarged diameter vias are centered around the metallines 205 as described above, wherein the enlarged contact area betweenthe vias and the metal lines includes three surfaces of the metal lines,consistent with the embodiment shown in FIG. 11A. In FIG. 12B, enlargeddiameter via openings 212 c are formed so as to extend below the lowersurface 214 of the metal lines 205, such that the contact area isincreased even further by including all four surfaces of the metal lines205. Extending the depth of the enlarged diameter vias 212 c can beaccomplished by over-etching the first low-k material 222 after anendpoint is detected. If the over-etch step uses an isotropic etchchemistry, the metal lines 205 can be undercut to produce the profileshown in FIG. 12B which exposes all four sides of the metal lines 205.The metal lines 205 can be secured at other locations in front of, orbehind, the cross-sectional plane shown.

The techniques presented herein are generic and can be used at any metallayer, to ease constraints on via formation, thus supporting metalstructures having a variety of different line widths. Without thebenefit of the structures and processes described herein, via aspectratios would continue to increase as the nominal metal line widthshrinks with each new technology generation. Thus, severing thedependence of via dimensions on metal line widths has importantadvantages and implications for future generations of integrated circuitdevelopment.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet areincorporated herein by reference, in their entirety. Aspects of theembodiments can be modified, if necessary to employ concepts of thevarious patents, applications and publications to provide yet furtherembodiments.

It will be appreciated that, although specific embodiments of thepresent disclosure are described herein for purposes of illustration,various modifications may be made without departing from the spirit andscope of the present disclosure. Accordingly, the present disclosure isnot limited except as by the appended claims.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. An interconnect structure for use in integrated circuits, thestructure comprising: a first metal line having a plurality ofperiodically spaced narrow portions, intermediate width portions, andwide portions, each of the portions being spaced apart with respect toone another and being located at regular intervals along the first metalline, the first metal line extending longitudinally in a selecteddirection; a second metal line vertically spaced apart from the firstmetal line by an inter-layer dielectric; and an enlarged diameter viaforming an electrical connection between the first and second metallines, the enlarged diameter via intersecting a narrow portion of thefirst metal line to provide direct contact between the enlarged diametervia and at least three different surfaces of the first metal line. 2.The interconnect structure of claim 1 wherein the inter-layer dielectricis a bi-layer film that includes a lower component made of a firstdielectric material having a first dielectric constant and an uppercomponent made of a second dielectric material having a seconddielectric constant, the first and second dielectric constants differingfrom one another.
 3. The interconnect structure of claim 2 wherein thefirst dielectric material is adjacent to the first metal line and has afirst dielectric film height substantially equal to a height of thefirst metal line.
 4. The interconnect structure of claim 2 wherein thedielectric bi-layer film includes one or more of silicon carbide (SiC),silicon carbide hydroxide (SiCOH), porous silicon carbide hydroxide(pSiCOH), or an OMCTS film.
 5. The interconnect structure of claim 1wherein the enlarged diameter via is an offset via that is alignedasymmetrically with respect to the first metal line.
 6. The interconnectstructure of claim 1, further comprising: a via landing pad formed ateach wide portion of the first metal line; a third metal line verticallyspaced apart from the first metal line in a direction opposite that ofthe second metal line; and a via forming an electrical connectionbetween the via landing pad and the third metal line.
 7. Theinterconnect structure of claim 1 wherein a contact area between thefirst metal line and the enlarged diameter via includes at least fourdifferent surfaces of the first metal line.
 8. The interconnectstructure of claim 1 wherein the enlarged diameter vias are insulatedfrom one another by an encapsulating, silicon carbide-based inter-layerdielectric.
 9. The interconnect structure of claim 8 wherein the air gaphas a width within a range of about 10-50 nm.
 10. The interconnectstructure of claim 1 wherein the first metal line is spaced apart fromneighboring metal lines by a pitch distance within a range of about10-65 nm.
 11. A method of forming enlarged diameter vias that intersectunderlying metal lines, the method comprising: forming an inter-layerdielectric (ILD) on top of the underlying metal lines; patterning asurface of the ILD with enlarged diameter vias having a characteristicdiameter that exceeds a width of the underlying metal lines; etching theenlarged diameter vias to an enlarged diameter via depth that extends atleast to a lower surface of the underlying metal lines; and filling theenlarged diameter vias with a metal liner and a bulk metal.
 12. Themethod of claim 11, further comprising extending the enlarged diametervias below the lower surface of the underlying metal lines and removingILD material underneath the underlying metal lines to create all-aroundvias.
 13. The method of claim 12 wherein removing the ILD materialunderneath the underlying metal lines includes performing an isotropicetching step.
 14. The method of claim 11 wherein etching the enlargeddiameter vias further comprises use of an advanced process controlsystem to determine an etch time from statistical in-line measurements.15. The method of claim 11 wherein forming the ILD includes forming abi-layer that includes two different ultra-low-k (ULK) dielectricmaterials.
 16. The method of claim 15 wherein an interface of the twodifferent ULK dielectric materials is used as an etch stop layer todetermine the enlarged diameter via depth.
 17. The method of claim 11,further comprising forming a tapered air gap, the tapered air gapextending downward to the lower surface of the underlying metal lines.18. The method of claim 11 wherein at least some of the enlargeddiameter vias are substantially asymmetrical with respect to theunderlying metal lines.
 19. A method of patterning a semiconductorwafer, the method comprising: rotating the wafer from an original waferorientation through a selected angle; exposing the wafer to lightthrough a lithography mask patterned with oblique features; andcounter-rotating the wafer through the selected angle to restore theoriginal wafer orientation.